The present invention relates to a filter for collecting combustible particulates containing carbon particulates as a main ingredient (hereinafter will be called "carbon particulates") in exhaust gases emitted from an internal combustion engine such as a diesel engine.
Conventionally, a ceramic honeycomb structure coated with .gamma. alumina or the like and carrying oxidizing catalyst has been used as the filter described above.
However, when the filter formed of the ceramic honeycomb structure is disposed in an exhaust passage of a diesel engine which emits a large amount of carbon particulates, a substantial amount of the exhaust gases passes through open passages of the honeycomb structure which extend in the same direction as the direction of flow of the exhaust gases. As a result, carbon particulates collecting efficiency of the filter becomes low.
In order to overcome the defect of the above described filter, a ceramic porous body provided with interconnected pores having large contact area with the exhaust gases has been proposed as the filter having excellent carbon particulates collecting efficiency.
However, this ceramic porous body is very brittle due to compression, vibrations or the like. And since the ceramic porous body has interconnected pores opening into the whole outer peripheral surface thereof, these interconnected pores must be closed so as to prevent the exhaust gases from leaking therefrom except for an exhaust gas inlet port and an exhaust gas outlet port.
In order to overcome the defect of the filter formed of ceramic porous body, a filter formed of ceramic porous body having uniform bulk density, provided with an outer wall of which cell density is large and uniform, and which is integrally formed with the ceramic porous body, has been proposed.
According to the filter provided with an outer wall, leakage of the exhaust gases can be prevented and mechanical strength is improved.
However, this filter still has defects to be solved. One of the defects is that mechanical strength of the border portion between the outer wall and the periphery of the porous portion is not sufficiently high. The other defect of this filter is that in the above described border portion, thermal-expansion coefficient largely changes due to sudden change of the wall thickness from the outer wall to the porous portion so that thermal shock resistance is very low.
Accordingly, one object of the present invention is to provide a filter for cleaning exhaust gases emitted from a diesel engine, which has high carbon particulates collecting efficiency and excellent durability.
Another object of the present invention is to provide a filter for cleaning exhaust gases emitted from a diesel engine, which is formed of a porous ceramic body provided with an outer wall preventing leakage of the exhaust gases and giving mechanical strength thereto.
Still another object of the present invention is to provide a filter for cleaning exhaust gases emitted from a diesel engine, which is formed of a porous ceramic body provided with an outer wall and which has improved thermal shock resistance.
A further object of the present invention is to provide a method for forming an outer wall having excellent mechanical strength and thermal shock resistance, on the outer periphery of the porous ceramic body.